A capacitor is a passive electronic component that is used to store energy in the form of an electrostatic field, and comprises a pair of electrodes separated by a dielectric layer. When a potential difference exists between the two electrodes, an electric field is present in the dielectric layer. An ideal capacitor is characterized by a single constant value of capacitance, which is a ratio of the electric charge on each electrode to the potential difference between them. For high voltage applications, much larger capacitors are necessary.
One important characteristic of a dielectric material is its breakdown field. The breakdown field corresponds to the value of electric field strength at which the material suffers a catastrophic failure and conducts electricity between the electrodes. For most capacitor geometries, the electric field in the dielectric can be approximated by the voltage between the two electrodes divided by the spacing between the electrodes, which is usually the thickness of the dielectric layer. Since the thickness is usually constant it is more common to refer to a breakdown voltage, rather than a breakdown field. There are a number of factors that can dramatically reduce the breakdown voltage. In particular, the geometry of the conductive electrodes is important factor affecting breakdown voltage for capacitor applications. In particular, sharp edges or points hugely increase the electric field strength locally and can lead to a local breakdown. Once a local breakdown starts at any point, the breakdown will quickly “trace” through the dielectric layer until it reaches the opposite electrode and causes a short circuit.
Breakdown of the dielectric layer usually occurs as follows. Intensity of an electric field becomes high enough to “pull” electrons from atoms of the dielectric material and makes them conduct an electric current from one electrode to another. Presence of impurities in the dielectric or imperfections of the crystal structure can result in an avalanche breakdown as observed in semiconductor devices.
Another important characteristic of a dielectric material is its dielectric permittivity. Different types of dielectric materials are used for capacitors and include ceramics, polymer film, paper, and electrolytic capacitors of different kinds. The most widely used polymer film materials are polypropylene and polyester. Increasing dielectric permittivity allows for increasing volumetric energy density, which makes it an important technical task.
Second-order nonlinear optical (NLO) effects of organic molecules have been extensively investigated for their advantages over inorganic crystals. Properties studied, for example, include their large optical non-linearity, ultra-fast response speed, high damage thresholds and low absorption loss, etc. Particularly, organic thin films with excellent optical properties have tremendous potential in integrated optics such as optical switching, data manipulation and information processing. Among organic NLO molecules, azo-dye chromophores have been a special interest to many investigators because of their relatively large molecular hyper-polarizability (b) due to delocalization of the p-electronic clouds. They were most frequently either incorporated as a guest in the polymeric matrix (guest-host polymers) or grafted into the polymeric matrix (functionalized polymers) over the past decade.
Hyper-electronic polarization of organic compounds is described in greater detail in Roger D. Hartman and Herbert A. Pohl, “Hyper-electronic Polarization in Macromolecular Solids”, Journal of Polymer Science: Part A-1 Vol. 6, pp. 1135-1152 (1968). Hyper-electronic polarization may be viewed as the electrical polarization external fields due to the pliant interaction with the charge pairs of excitons, in which the charges are molecularly separated and range over molecularly limited domains. In this article four polyacene quinone radical polymers were investigated. These polymers at 100 Hz had dielectric constants of 1800-2400, decreasing to about 58-100 at 100,000 Hz. Essential drawback of the described method of production of material is use of a high pressure (up to 20 kbars) for forming the samples intended for measurement of dielectric constants.